Steering Issue

[rushers]

I have a 2.9m standard SIB with an inflatable keel, 5HP yamaha outboard. When on the plane the bow of the boat has a rapid left to right "S shaped" movement. I think the correct term is yaw. This is not chine walking as I understand it. The nose/bow seems to have a rapid left/right, left /right movement. What is the cause? Any remedies? I was travelling with the current in very little wind.

[Erin]

I doubt you would ever get true chine walking on a sib. The hull just isn't hard enough and their isn't a deep enough vee to make it happen. However, I suspect you are suffering something akin to it which could possibly be reduced by trimming the engine in a little to lower the bow.

[jyasaki]

Is it bad enough that it causes a problem (i.e. you have to fight the changes, or it might get bad enough to be unsafe)? You can get a lot of feedback through the hull in less-than-glass-smooth water, and a little meaningless twitch may still be felt.

That said, changing weight around will likely help, as will slowing down in that particular condition.

Just thought of another possibility: are you talking about hull steer as you run faster than a following sea?

jky

[rushers]

Thanks for the replies Erin and Jyasaki. I think I probably had some bow/hull steering and the answer may be to to trim the engine out? My motor was fully trimmed in so I guess too much bow down increasing the drag and causing the instability. It felt pretty unstable on the plane ( the twitching of the bow was too rapid too try and make corrections) and seemed to be plowing at slow speeds before this. If the motor was not properly centered could this also add to the problem? Jyasaki I was in a river mouth pretty flat with small chop traveling with a following current from a pushing tide.

[jyasaki]

The twitching was probably due to chop hitting the hull at slightly different angles (from varying waves.) I would guess that it's not really anything to worry about; just something that takes some getting used to. My SIB used to feel like it was a small levitating disk being bounced side to side and fore-aft by the small chop. Actually, in certain conditions my larger RIB gets a similar ride.

Trimming up may help; it will bet the bow up a bit which will put the force more towards the stern, which should be a little more stable side-to-side.

jky

[rushers]

I am pretty green to this - it felt very nervous and twitchy, I guess with more experience I may smile at this post some time in the future . I will fiddle with the trim and see what happens. What effect will an poorly centred motor cause?

[jyasaki]

Often motors are mounted slightly off-center to counter the rotational torque of the prop, which tends to turn the motor, and hence the boat, at high power settings.

I suspect that a motor mounted off-center will simply cause the boat to steer to one side. Never tested it though (all my boat motors have been bolted on.)

jky

[willk]

Can I ask where are you sitting in this SIB and what else have you O/B (and where is it stored? i.e. fore/aft)

[frankc]

If a motor is tilted too close to the trasom, you can experience bow steer which may be the twitchy you noticed. That can be dangerous and result in the boat tending to spin around uncontrollaby. The cavitation plate should be parallel to the surface of the water at speed or tilted slightly out so that the boat rides slightly bow high.

As far as centering the motor on the transom, it should be centered. The only reason for it to be intentionally offset would be to make up for a defective hull. If you see an outfit with the motor not centered, that should be a red flag that there is something wrong with the hull.

[rushers]

Willk I had nothing in the boat except my "massive" 65 kg frame . I was sitting as far forward as I could within the limits of remaining "attached" to the tiller about 1/2 way in a 3m SIB on the port side - this was the best way to get her up on the plane. The only luggage on board was my safety gear and a 3 kg anchor stored aft in front of the motor.

[frankc]

Take a look at runabouts with steering wheels and you will note that the helm is on the starboard side of the boat. This is to counteract the prop rotation which tries to lift the starboard side of the boat. Sitting on the port side of the boat increases the list to port. Sit on the starboard side which is the proper way to steer a small boat.

Once a boat is on plane, the weight should be shifted to the stern in order not to cause bow steering. If the boat runs with the front of the keel too deep, you can experience oversteer. Hit a wave wrong and you may flip suddenly.

Something like a Dole fin or Stingray on the cavitation plate should be helpful in your case, as it helps keep the bow down.

One question is whether or not you are trying to shorten the time to plane by tucking the motor in too close to the bottom of the transom? Many boats take a little time to reach planing speed. With power tilt, many tuck in the motor to assist in getting out of the hole and then trim out as speed increases. We generally do not have the luxury of power tilt on small motors.

I have been running small helm steered boats for 60 years now, and you may be in for a learning experience if you allow bow steering. If you do, expect to go swimming sooner or later

[rushers]

Hi Frankc
Thanks for the reply , it is truly amazing that I can sit more than 10 thousand miles away and tap into the experience of someone with 60 years of doing this. You gotta love the internet .
I have no doubt that you are correct as I had originally set the boat up on the 3 rd hole of the manual tilt assembly as this was the best position to get the cavitation plate parallel to the floor. Unfortunately I had sent the trailer in for some mods and the tilt pin was "lost" , so I could only run fully trimmed in. New tilt pin arriving tomorrow so I will do a few more "river trials" on the weekend. I will try the starboard side, although as a righty it just seemed natural to have the tiller in my right hand ( the throttle and steering were however counter-intuitive this way, so starboard side may be better). I most certainly would like to avoid swimming - we have some critters with big teeth in our river mouths over the winter and early summer .

[captnjack]

Agree with the others, trim the motor out. At least 2 probably 3 holes. Bow steering is cause by having the forward part of the hull too far in the water. You want some bow lift. So you definitely don't want a doel-fin type thing for now which would press the bow even further down into the water.

[frankc]

We use a Doel fin on our 25 HP running a 14 ft fiberglass boat. We porpoise like mad at full throttle without it. As it is on the cav plate, it does not force the bow down except when accelerating and then it really just helps with stern lift. We lose about 2 mph at top speed in calm water, but it still runs well. It is a dream in rough water and probably as fast or faster than running the motor without a fin. Boat stability is better with the fin.

If the tilt pin was lost, then it is understandable to try the boat without it. Just be careful when running like that as bow steer causes bad things to happen awfully fast. I have had the tiller do scarey things when trimmed like that. Having our old offshore rig left with the trim down and then hitting the throttle would always get to you if the water was rough. The boat would try to slew to the side before you realized it. Got the gray hair to prove it

Once you get the initial setup correct, you will find a boat is an enjoyable thing. I love the water.

Once you get accustomed to handling the tiller with your left hand, you will wonder why no one told you about that before. The side to side balance will be correct and the boat will handle much easier, especially in turns. Let me know what you think after getting a few hours on the boat while controlling from the starboard side.

I still am in awe of the internet. On a knife forum from England, people from Uragray to other distant points have been very helpful to me.